1. Field of the Invention
The present invention relates to an intake duct as a passageway adapted to supply air to an engine, and more particularly to an intake duct capable of reducing noise during use.
2. Description of the Related Art
Intake systems used with motor vehicle engines generate noise in conjunction with the intake of air. This intake noise is offensive to persons in the motor vehicle, particularly when the engine is running at low speeds. Conventionally, side branches and/or resonators have been provided in such intake ducts to reduce the noise at a specific frequency, calculated, for instance, in accordance with the Helmholtz's resonance theory.
However, the side branches may be as long as about 30 cm, and the resonators may have a volume as great as 14 litters. To install these devices in the engine compartment, increased space is needed which necessarily decreases the amount of room available for installing other parts.
Japanese Utility Model application laid-open No. Sho 64-22866 discloses an arrangement where orifices are provided in an intake duct, and intake air is throttled at the positions of the orifices; thereby reducing intake noise. By throttling the intake passage in this manner, the acoustic mass increases, and consequently, the intake noise of the engine at low frequencies can be reduced.
Japanese Utility Model application laid-open No. Hei 3-43576 discloses a device for reducing intake noise that includes two intake pipes, and a valve which selectively opens and closes the upstream side of one of the intake pipes in accordance with the an operating mode of the engine.
An intake duct shown in FIG. 49 for example, corresponds to the intake duct disclosed in Japanese Utility Model application laid-open No. Hei 3-43576, and includes a first intake pipe 1000 of a small diameter and a second intake pipe 2000 of a larger diameter. In the second intake pipe 2000, a valve 3000 is pivotably provided. A pivot shaft of the valve 3000 is connected to a center of a disk-like cam 4000, and a spring 5000, one end being fixed to a vehicle body, is secured to a periphery of the cam 4000. The spring 5000 is arranged such that the biasing force thereof is reduced to a minimum when valve 3000 fully closes the second intake pipe 2000.
With the intake duct thus arranged and the intake pressure of the second intake pipe 2000 less than a predetermined value, valve 3000 fully closes the second intake pipe 2000 so that air is drawn through only the first intake pipe 1000; this reduces intake noise across low frequencies. When the intake pressure exceeds the predetermined value, valve 3000 pivots against the biasing force of spring 5000, which is transmitted through cam 4000, to increase the opening area of the second intake pipe 2000 in proportion to the intake pressure, thus enabling a sufficient amount of air to be supplied.
The above-described method of throttling the intake passage, however, produces a problem when the engine runs at a high--speed since the amount of intake air is insufficient and lowers the engine output.
With the device disclosed in Japanese Utility Model application laid-open No. Hei 3-43576, there is a state where valve 3000 is held between a fully closed position and its fully-open position to prolong the time period the second intake pipe 2000 is half closed. In this case, the problem is that since the acoustic mass is small, the intake noise is large even though the engine is running at a low speed, resulting in booming noise of low frequencies entering the passenger compartment of the motor vehicle.
Furthermore, Japanese Utility Model application laid-open No. Hei 3-43576 discloses the use of an electronic control circuit, an electromagnetic valve, and a diaphragm actuator, or the like, for operating a valve member. The use of these parts increases in the total number of parts used, and also increases the complexity of the device, both of which are less favorable in terms of production costs.